Breastmilk contains a whole bunch of other stuff that's beneficial, not just the growth factors that help gut flora and the digestive system to develop. It contains antibodies, anti-virus, anti-parasites, hormones, enzymes .. etc: all things that help to protect the child from illness and to develop a healthy immune system.
Formula is still a very healthy alternative for babies whose mothers choose not to breastfeed or cannot breastfeed. Nonetheless, formula does not contain all of the things that breastmilk does. Even so, despite their differences the benefits of breastfeeding and formula feeding continue until at least one year of age. Cows milk and other milks other than breastmilk and formula do not have the correct balance of nutrients or fats for a growing baby. It's recommended that if you stop breastfeeding before age 1, you switch to formula and continue to give formula until age 1. Then only after that point can you switch to cows milk or another milk (e.g. soy). Solid foods are not even close to sufficient to providing the nutrients a 0-1 year old needs, particularly as many infants will not even eat solids (they refuse) or will only eat small amounts or will only eat specific foods. There's a saying that "food under one is just for fun" and this is because the primary source of nutrition should still be from breastmilk or formula up until this age.
Aside from the nutritional benefits of breastmilk, there are many benefits of breastfeeding for the mother as well (such as reducing your risk of some types of cancer, preventing postpartum depression..). Breastfeeding your baby, or feeding your baby formula or breastmilk through a bottle also helps to strengthen the bond between parent and child. If you just stopped feeding your baby milk (whether breastmilk or formula) because their gut had closed and they were ready for solids, that would not be healthy at all.
